1. Field of the Invention
The present invention relates to the field of transmitting a bitstream over a communication medium. More specifically, the invention relates to the real-time transmission of digital signals into transport protocol packets.
2. Description of the Invention
In order to transmit encoded digital data from one system over a communication medium to another requires carrying the bitstream as payload data within the payload section of transport protocol packets. This process of inserting data into another protocol's packet is known as “encapsulation” and involves “segmentation” of the bitstream at the source end into packets. Thereafter, the segmented bitstreams are reassembled at the receiving end. Thus, the encapsulation process allows multimedia data to transmit from one location to another over a packet network.
A Real-time Transport protocol (RTP) is typically used to transmit multimedia data on a network such as the Internet. The data is compressed using a specific compression technique and the compressed data stream is broken down into smaller packets for transmission over the communication medium. This process is referred to as “packetization” and the reverse process, i.e., assembling network packets into a continuous byte stream, is called “depacketization”. One significant impediment to reliable multimedia streaming on packet data networks is packet loss. Packet loss occurs due to limited buffering and processing capabilities of network nodes, and to a lesser extent, due to bit errors in physical links. Referring to FIG. 1, many real-time applications employed over the Internet uses negative acknowledgment messages sent by the client to determine when a retransmission is required. As shown in FIG. 1, if packet loss occurs during transmission, the client in a streaming session sends a negative acknowledgment (NACK) in response to each lost packet. The NACK packet contains the sequence number of the packet that needs to be retransmitted by the server. As the data stream is typically prepacketized before the transmission, the server can always retrieve the necessary packet from storage (e.g., a file) and retransmit to the client.
FIG. 2 illustrates a conventional structure of a packetized stream used for retransmission. Using static header information, both the streaming header and packet data can be retrieved from one location inside the file. Here, knowing the offset OFFi of packet i within the corresponding file FILEi and the size Si of the packet, the server can easily retransmit packet i upon receiving a NACK for packet i. Therefore, the server needs to know only the three sets of related information (FILEiOFFi, Si) about each packet, which are often stored in a file called the meta-file of the session. However, in the event that the server implements real-time (i.e., on-the-fly) packetization of the multimedia stream, the support for retransmission becomes more difficult using this conventional method as each streaming header is static and does not change when the server transmits it to the client.
Real-time packetization is necessary, for example, if the server transmits data from a live source (i.e., live TV broadcast) or if the server needs to alter the stream in the middle of a transmission (i.e., adapt the size of each picture to the available bandwidth). In real-time packetization, the server arbitrarily decides the composition of each packet at the time of transmission; thus, the server has no way of reconstructing the header information from a sequence number contained in the received NACK from the client. Thus, if the conventional method is utilized in real-time packetization, the support for retransmission is unattainable as there exists no prior knowledge of header contents of each sent packet. Another disadvantage of the conventional method is that the server is usually forced to transmit packets whose size is less than the maximum packet size permitted in the underlying protocol. The consequence is wasted bandwidth from leaving portions of the packet payload unused.
Accordingly, there is a need to provide a more efficient method and apparatus for providing real-time packetization to transmit multimedia data streams with a lower overhead and to permit the retransmission of lost packets, while supporting real-time packetization.